Modeling dynamical phonon fluctuations across the magnetically driven polaron crossover in the manganites

TL;DR

This paper models lattice fluctuations and phonon dynamics in manganites near the polaron crossover, revealing how thermal effects influence phonon lineshapes and electronic properties, with comparisons to experimental neutron scattering data.

Contribution

It introduces a Monte Carlo-based adiabatic model capturing thermal polaron formation and phonon fluctuations in manganites, linking microscopic dynamics to experimental observations.

Findings

Phonon lineshapes vary across the ferromagnet to paramagnet transition.

Thermal fluctuations induce short-range polaron correlations affecting spectral functions.

Predictions for phonon behavior in insulating phases are provided.

Abstract

We investigate the dynamical structure factor associated with lattice fluctuations in a model that approximates the manganites. It involves electrons strongly coupled to core spins, and to lattice distortions, in a weakly disordered background. This model is solved in the adiabatic limit in two dimensions via Monte Carlo, retaining all the thermal fluctuations. In the metallic phase near the polaronic crossover this approach captures the effect of thermally induced polaron formation, and their short range correlation, on the electronic spectral functions. The dynamical fluctuations of the optical and acoustic phonon modes are computed at a `one loop' level by calculating the electronic polarisability in the the thermally fluctuating backgrounds, and solving the phonon Dyson equations in real space. We present phonon lineshapes across the ferromagnet to paramagnet thermal transition and correlate them with changing electronic properties. We compare our results with inelastic neutron scattering data on the metallic manganites, and also predict what one may find in the more insulating phases.